1. Field of the Invention
The present invention relates generally to a device and method for binarizing an image, and in particular, to a device and method for preprocessing an input image into a binary signal before recognizing characters in the input image.
2. Description of the Related Art
Generally, a preprocessing operation is performed to recognize image characters. “Preprocessing operation” refers to an operation of processing an image before recognition of characters in the image. The image preprocessing operation can include an operation of deciding whether or not an input image is appropriate for character recognition, an operation of correcting a skew of an object in an input image, an operation of properly correcting a size of an input image, and an operation of binarizing an input image so that characters in the input image can be recognized.
FIG. 1 is a block diagram illustrating the structure of a conventional binarization device for binarizing an image. The binarization device shown in FIG. 1 uses a quadratic filter. The quadratic filter is disclosed in a reference entitled “A Polynomial Filter for the Preprocessing of Mail Address Images,” by P. Fontanot and G. Ramponi et al., Proc. 1993 IEEE Winter Workshop on Nonlinear Digital Signal Processing, Tampere, Finland, January 1993, pp. 2.1-2.6, the contents of which are incorporated herein by reference.
Operation of the quadratic filter will now be described. A first threshold selection part 11 calculates a first threshold Th1 used for classifying pixels of an image into character pixels and background pixels. A mean computation part 13 classifies the pixels of the image into character pixels and background pixels on the basis of the first threshold Th1, and computes their mean values. A normalization part 15 converts pixels of the input image into values close to ‘1’ or ‘0’ using mean values of the character pixels and the background pixels, output from the mean computation part 13. It is assumed herein that the normalization part 15 converts the character pixels into a value close to ‘1’ and the background pixels into a value close to ‘0’. A quadratic operation part 17 performs the operation of enhancing edges of the normalized pixels using relations between a given central pixel and its neighboring pixels with respect to the respective pixels. A denormalization part 19 performs the operation of denormalizing the edge component-enhanced pixels output from the quadratic operation part 17 in the range of their original pixel values. A second threshold selection part 21 calculates a second threshold Th2 used for classifying the denormalized pixels into character pixels and background pixels.
Simple binarization part 30 converts the pixels output from the denormalization part 19 into two specific brightness values on the basis of the second threshold Th2.
When such binarization is performed on the entire image photographed in an irregularly lighted situation, however, with a shadow thrown thereon, binarization performance deteriorates undesirably.